[IEEE IEEE 34th Annual Spring Reliability Symposium, 'Reliability - Investing in the Future' - Boxborough, MA, USA (1996.04.18-1996.04.18)] IEEE 34th Annual Spring Reliability Symposium, 'Reliability - Investing in the Future'. - Manufacturing process study and certification

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  • Manufacturing Process Study and Certification Dr. M.Elbert, R.Howe

    Digital Equipment Corporation

    INTRODUCTION The importance of product quality and reliability, time to market, cost and customer satisfaction continue to grow as the competitive battlefield expands. Manufacturing processes have the most important role in producing products that meet cus- tomer expectations and required level of product quality/reliability. Why '? Average industry data indicate that manufacturing process problems represent approximately 60 % of all product problems, and material and design product problems represent 30 % and 10 % respectively.

    This paper presents our approach in the area of manufacturing process study and certification program aimed to assure that the manufacturing process consistently/repeatedly produces prod- ucts that meet required quality and reliability goals. This paper presents principles, plans, processes, structure, guideline, performance measure- ments and criteria necessary to achieve and ma.intain manufacturing process requirements and certification. Utilization of FMEA (and RPN), Process Audit, Process Perfomance / Capabil- ity Study (Cpk, PPM - defect rate, DPMO), Statistical Process Control (SPC, control charts) are discussed.

    The key to an effective manufacturing process is to have measurements of process perfom- ance/capability that reflect customer expectations, and quantify the results of work performed by the process.

    Such aspects as the differences between process performance and process capability, different conventional statistical theory tools to define acceptance tests approaches and sample sizes are discussed in the paper.

    OBJECTIVES OF PROCESS CERTIFICATION

    The overall goal of the process certification program is on assuring that manufacturing opera- tions consistently produce products that meet required quality and reliability goals. Process certification provides the organization with a stable, measurable manufactukg process that meets customer expectations. A systematic process certification is a way to move from an appraisal-type philosophy into a preventive-type philosophy.

    The ultimate long-range objective for process certification is provision (and demonstration) that the process is capable of producing zero process defects at sustained line output levels for a period of time. It implies the need for never-ending improvement. The Cpk level > 2.5 can be considered as a goal for provision of zero defect.

  • DEFINITIONS The general terms and definitions used in this paper are presented below. Manufacturing Process is unique combination of machine, tools, methods, materials, and people used to manufacture or fabricate a product. This combination can apply to the entire process or to the individual operation. For the various manufacturing groups (Modules, Sys- tems, Software) the process can be defined as combination of operations. For example, for module manufacturing the process is defined as each workstation within the process. Process Certification is assuring of (1) the required organizational and functional structure, and (2) required performance of the complete manufacturing process.

    (1) Organizational and Functional Structure of Quality System. A certification process should demonstrate that all the necessary responsibilities, training, documentation, measurements, controls, etc. are in place to ensure that the manufacturing process can produce the required level of product quality.

    ( 2 ) The required level of performance should be measured and demonstrated. Also, the process shall demonstrate that it can repeatedly produce the required level of product quality.

    Certification applies to the complete process and to a single operation. For the complete process to be certified, each operation in the process should be certified. Qualification Process is the process defined to ensure that requirements specified for a prod- uct are met. The qualification process consists of a sequence of environmental, mechanical, electrical, functional, software, regulatory, safety and other tests which are conducted in accor- dance with the requirements of Qualification Strategy.

    The Process Control is defined as a system for measuring and analyzing of the manufacturing processes. The process control system contains three major elements - data system, process perfomance/capability measurements/quantification and a feedback loop through which it compares with requirements, and act on difference.

    Statistical Process Control (SPC) is the application of statistical techniques for measuring and analyzing of the processes (control). Statistical process control verifies the stability of the process and homogeneity of the product. Process PerformanceKapability is the capacity or ability to reproduce product characteristics or degree of quality attribute.

    Process performance and capability are measurable properties of the process.

    In general, process capability is expressed in terms of the Cp and Cpk measures where Cp is a measure of product uniformity and Cpk is a measure of product uniformity and target centered- ness. Performance Index Cpk is estimated by:

    I x - L S L x - U S L I

    1 I 3s 3s I

    Cpk = fie { __------_____ _____________

    where LSL and USL are the lower and upper specification limits respectively, and s is stan- dard deviation.

  • Process performance is not the same as process capability. The difference involves the assump- tiodstate of statistical control. If a process is in statistical control, than the measure of process performance results in determining (inherent) process capability. It is important to distinguish between the process in a state of statistical control and process that is meeting specification. A state of statistical control does not necessarily mean that the product from the process conforms to requirements or specifications. On another hand, the process can meet the product tolerance and requirements even with a nonSPC status.

    Fig. 1: Process Certification Whv Process

    Provfdeorganlzatlor &functlonal structure

    ldentlfyparamateffi crltical to process I RPN Develop mrrectlve actlons Fall safe operatlons

    VaildateSPC rc YES P ~ c a p a b i l l ~ S p c SRKty

    Defineprocess

    ValldateFMEA capability

    Achievecertiflcatlon Provldedefect alterla

    Maintain defect aiteria

    Achleve 0 defect

    f

    All U 0 complete wlth approvals

    elements

    ldentlfled

    RPN Pareto for all

    Controlsystem

    SPC demonstrated All Cpk measured All DPMO and defect rate measured ConlJoi charwtlistograms developed

    Odefect demonstratlon on =we

    Sustain output level

    Sustain output level pproach longterm

    Change requirements - DPMO defect rate - benchmarklng

    Metrics

    Provislon of all requirements (1 ooO/)

    RPNPareto NocritldFMEh elements with (>25 RPN)

    Ope~tlOnS Percent fail safe

    Percent elements with Cpk > 20 All Cpk > 1.33 All PPM < 63

    Demonstrate required DPMO &defect rate

    Percent processes cemed by production line

    Demonstrate requlred DPMO on mminuous bases

    Cpk > 2.5 PPB- 1.0

  • CERTIFICATION STRUCTURE / GUIDELINE The flow diagram of the process certification process is presented on Fig. 1.

    Process certification is an iterative process. Results of activities in any phase may require cor- rective actions, feedback loops and improvements of the previously developed phase.

    The entire process ceftification structure is presented in the Process Certification Road Map (Table 1). The structure contains three fundamental components - Vertical structure, Horizontal structure, and Measurements which are used to measure the result and fulfillment Qf certifica- tion activities in each particular phase.

    Table 1

    Process Certification Road Ma

    CFRTlF lCATlON SYSTFM APPl CATION fWHFN 8 WHFRF") MEASUREMENT - flAQuS2 STAGE 1 STAGE 2 SORWARE Each Operation Complete Process

    Method Material Operator Test 8 Process EquipmenVCombination

    PROCESS AUDIT > IS0 9000 & Speufic Check Lists

    > Responsibility and Management > Measureable Goals and Customer

    Expectations z Documentation > Row Diagram > Control (SPC), Data System and feedback

    > Corrective actions > Training of production personnel1 > Operational requirements (Envrionments,

    FMEA, Producttraceability, etc.) > Output aaxptabi l i

    > Identify potential failure modes and cMuses, RPN Identify corrective actions (failsave technique)

    > Define critical product parameters and process variables

    > Idenbfyfimprove control

    > Data collection system

    z Process perfomancekapabilii study z Pareto analysis > Feedback adjustments > Design of experiment

    systems

    FMEA

    PROCESS CONTROL

    Statistical process control

    PROCESS ACCEPTABILITY PROCESS

    PROCESS IMPROVEMENT MONITORING/MAINTENANCE

    PROVISION OF ALL ITEMS IN CHECKLISTS

    RPN, LIST OF CORRECTIVE ACTIONS,

    CRITICAL FACTORS

    SPC Cpk, DEFECT RATE, DPMO

    FMEA VAUDATlON

    ZERO DEFECT for SAMPLE Cpk, DEFECT RATE, DPMO

    on Continuouse Basis Cpk. DPMO BENCHMARKING

  • Vertical Structure. The vertical structure defines specific certification subprocesses/ activi- ties / phases which represent "whats"of certification. The vertical structure contains the follow- ing major activities: Process Audit, Failure Mode and Effect Analysis (FMEA), Process Control, Process Performance (Capability) and Statistical Process Control (SPC) Study, Proc- ess Acceptability, Process Monitoring and Maintenance, and Process Improvement. Horizontal Structure. The horizontal structure defines implementation / application of certi- fication subprocesses / activities which represent "whens and wheres" of certification. The complete process or separate operation can be represented as any specific combination of (1) Machines, methods, material and people, and (2) DZferent manufacturing subprocesses (Modules, System, Software).

    The intersection of the vertical and horizontal structures represents combinations/blocks of dif- ferent certification subprocesses. The operator cerdfication, equipment calibration and main- tenance, provision of tracebility of the product are examples of specific certification subproc- esses.

    Measurements (see Fig. 1, Table 1) defines metrics which are used to measure the result / ful- fillment of certification activities in each particular phase/subprocess, and which are used to exit this phase and enter the following phase of certification activities.

    CERTIFICATION ACTIVITIES / PHASES - "WHATS" PROCESS AUDIT

    The objective of the audit is to assure that the required organizational and functional structure of the manufacturing process is provided. The systematic process audit is recommended to be performed twice per year. The audit should be repeated if a major change occurs in the process. Process certification in this stage is achieved through compliance with the audit checklists. The process audit consists of two phases: (1) compliance with IS0 9000, and (2) compliance with specific company requirements. Two Process Certification Checklists are recommended: IS0 9000 and Detailed /Specific Checklist.

    The process audit is organized into the following categories: a. Responsibility and management b. Measurable goals and customer expectations c. Documentation d. Flow Diagram e. Training of production personnel f. Equipment and Tools g. Operational Requirements (including proper environment, FMEA, process tracebility, etc.) h. Control , data system, feedback i. Process SPC and Capability j. Corrective actions k. Achieving Certification.

  • Table 2 DETAILED/SPECIFIC CHECKLIST - EXAMPLE

    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . mZE: SYSTFCMPROCESS CERTIFICATION PAGE .............................................................................

    ELEMENT : STAGE 2 Description POOR MARG. -----

    PACK Are Ioose-piccc locations spccified?

    mZ~aGzniz&m&r----- E s B f l T s t match thc rack I-$-- - H F t K i ~ ~ ~ r X i Z i Z n g u n i i eliminated (arc parts picked !o a sinde box?)?

    0 0 0 0

    0 0 0 0 0 0 0

    0 0 0 0

    0 0 0

    0

    0

    0 0 0 0 0 0

    0 0 0 0

    0 0 0 0 0 0 0

    0 0 0 0

    0 0 0

    0

    CERTIF.

    0

    0 0 0 0 0 0

    0 0 0 0

    0 0 0 0 0 0 0

    0 0 0 0

    0 0 0

    0 . - - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    The Detailed / Specific Checklist is used to provide and conom specific elements and require- ments of the complete process and each operation. Use of the checklists should emphasize the

  • identification of elements needing improvement. Specific checklists control many specific de- tails of operations. Examples of Detailed / Specific checklists are presented in Table 2. The documented evidence of compliance through systemic audits for all items should be pro- vided and satisfactory before the CeaLfication of this phase is granted. No any score/rating is recommended.

    FAILURE MODE AND EFFECT ANALYSIS ( FMEA). Next phase of certification process is Failure Mode and Effect Analysis. The major objective of M A in this phase of process certification is identlfy potential critical processes/operations and product elements required improvement / corrective actions. FMEA classifies and rank each potential failure mode according with risk. Risk Priority Num- ber (RPN) is used as a quantitative measure of a combined effect of Severity (l), Priority of occurrence (Z), and Likelihood of the problem Detection (3) .

    RI" = (1) x (2) x (3) RPN order is (1-1000). RPN critical value/threshold is usually 100-125 (this value is condi- tional and depends upon an approach taken to measure severity, occurrence and detection). Corrective actions are developed and prioritized based on FU". Problem solving approaches are used in this phase to develop corrective actions. Wide range of corrective actions shall be applied. Special attention shall be put on utilization of fail-safe techniques that do not allow de- fects to be passed to the next operatiodwork station. A constant improvement structure shall be implemented in this phase to drive RI" number to RPN = 25 level.

    The manufacturing process control can be modified based on the results of FMEA. Also FMEA provides the analysis of critical factors and correlation of process variables with prod- uct results. Cause and effect (Fishbone) diagrams are used for this purpose.

    Details FMEA (objectives, template, etc.) are presented in Table 3.

    Several metrics are used to measure the result of the FMEA and to exit this phase: the RPN value and RPN Pareto Analysis, percent distribution of critical factors with different values of RPN (50, 100, 125, etc.), a list of problems with their prioritization (Criticality List), list of recommended corrective actions with the final RF" value, list of fail-safe solutions.

    PROCESS CONTROL AND PROCESS CAPABILITY Process Capability Study. The major purpose of process capability study is to discover whether a process is in a steady state (state of statist...

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